In the theatre, plays are performed more or less as written. Producers and directors do their best to be faithful to a playwright's words, setting, plot, characters, and theme, particularly if the playwright is still alive. Even if a playwright is dead—like William Shakespeare, for instance—and even if a play is being adapted or otherwise subjected to an experimental production, producers and directors still want their production to have at least some resemblance to the original play.
Screenwriters learn very early in their careers that unless they're also the producer and/or director of a film, or a well-known screenwriter who is able to control to some extent what happens to their screenplay, once they sell their screenplay, it's not their baby anymore. From the moment they sell their screenplay, they have little or no control of what happens to it.
Producers have been known to buy a screenplay, keep the title, and discard the rest of the script, or have the script completely rewritten by other screenwriters.
Plays and screenplays are totally different forms of dramatic writing. A play script and a screenplay look nothing alike. The script of most plays consists primarily of dialogue, with little or no stage directions. Most screenplays contains a great deal of directions—settings and setting changes, character descriptions and character action, and camera angles and movement in "shooting scripts"—along with the dialogue.
Shakespeare wrote plays, not screenplays. He wrote plays to be performed on stage for an audience of his fellow Elizabethan and Jacobeans. He had no idea, no thought, no notion that his plays would one day be turned into screenplays, recorded on film or in electronic form, and presented as entertainment to the modern movie-going public.
In film-making, a screenplay is simply a frame of reference, a point of departure. The script of a play being made into a screenplay for a film is also simply a frame of reference. This applies even to the plays of William Shakespeare.
Converting a play into a screenplay, and then into a film, requires a seemingly endless number of decisions. There are so many differences between plays and films in concept, writing, and realization that it's very nearly futile to try to compare one to the other, or to compare part of one—the characterization of Shylock, for example—to the other.
Michael Radford, the director of the 2004 film version of The Merchant of Venice starring Al Pacino, also wrote the screenplay. Radford was responsible not only for the choices that needed to be made between Shakespeare's play and his screenplay, but also the decisions to be made in putting his written screenplay into cinematic form.
Radford decided to add a backstory for Shylock at the beginning of the film which doesn't exist in Shakespeare's play. He shows Shylock being insulted, mistreated, and even spat on by Antonio and other in the streets of Venice.
This persecution of Shylock gives the audience a frame of reference for the rest of the film of Shylock as a victim, rather than as the vengeful, hate-driven villain in Shakespeare's play.
Radford's characterization of Shylock occurs at the very beginning of the film, whereas Shylock doesn't appear in Shakespeare's play until the third scene, when Shylock is already actively involved in negotiations with Bassanio about a loan for three thousand ducats.
Once Radford decided that Shylock was to be portrayed primarily as a victim—although he's clearly still villainous in some ways in the film—Radford then had to make decisions through the rest of Shakespeare's play that conform to and support this particular interpretation of Shylock's character.
What the audience to Radford's film sees is not necessarily exactly what Radford intended them to see. Some scenes might have been shot even as Shakespeare wrote them, but for one reason or another were edited out of the final version of the film. The order of Shakespeare's scene were certainly changed, but the order of scenes in the film might also have been changed from the way Radford arranged them.
In all, the changes that Radford made to Shylock, along with the changes made by those who put the film in its final form, are based on Radford's personal interpretation of Shylock, and on the changes to Shylock's character—whether minor or significant changes—that were necessitated by the differences between a stage production of The Merchant of Venice and a film representation of the same play.